1. Field of the Invention
The present invention relates to an image forming apparatus performing image formation in electrophotography.
2. Description of the Prior Art
Conventionally, a photosensitive drum serving as an image carrying body, and a developing roller for developing an electrostatic latent image formed on the photosensitive roller are used in an image forming unit of an image forming apparatus making use of electrophotography and typified by printers, facsimiles and copy machines. In particular, in recent years, it becomes further important to accurately perform drive transmission of the photosensitive drum and developing roller so as to eliminate variations in speed, such as rotational fluctuation in order to obtain a stable image since output for color printing necessitates accurate color registration of toner images in respective colors.
Here, one of the causes for uneven rotational driving is resulted from a gap between gears engaging with each other. A gap (backlash) being formed between gears engaging with each other to make engaging-motion smooth, such cause is based on the fact that in the event of a change in drive transmission torque and a rapid change in rotational speed of a motor, tooth faces in meshing contact with each other to effect drive transmission separate from each other temporarily in the space of backlash to cause drive transmission to be suspended temporarily, with the result that rotational transmission speed intermittently fluctuates. This fluctuation leads to uneven rotation.
Therefore, in order to reduce such uneven rotation, it is important not to use more gears, which are drive transmission means, than necessary. Furthermore, it is particularly essential not to use any intermediate gear or gears serving only as occupying distances between shafts.
With an image forming apparatus, when uneven rotation arises in a drive transmission system, contraction and elongation generates in pitches in a paper feed direction, and in particular, with an image forming apparatus for color printing, color slurring generates to deteriorate color reproducibility.
Next, conventional drive transmission mechanisms for a photosensitive roller and a developing roller will be described.
Here, FIG. 4 is a perspective view showing an essential part of a conventional drive transmission mechanisms for a photosensitive roller and a developing roller. FIG. 5 is a front view showing the essential part of the conventional drive transmission mechanisms for the photosensitive roller and developing roller.
Drive transmission means consists of two systems of driving trains, that is, a train of drive transmission means called a developing roller driving train and comprising a developing roller driving motor 28, a developing roller 16, and drive transmission means such as several gears and pulleys, and a train of drive transmission means called a photosensitive roller driving train and including a photoreceptor driving motor 29 and a photosensitive roller 7.
As shown in the drawings, the developing roller driving motor 28 and photoreceptor driving motor 29 are mounted on a plate 27. A rotating shaft of the developing roller driving motor 28 extends through a hole formed in the plate 27, and securingly mounts a developing motor gear 32 to an end thereof. In addition, a developing roller driving gear 33 is rotatably mounted on the plate 27. Furthermore, an intermediate gear 35A is rotatably mounted on the plate 27 to engage with the developing motor gear 32 and the developing roller driving gear 33. In addition, the developing roller 16 for developing an electrostatic latent image on the photosensitive roller 7 is connected to the developing roller driving gear 33 through a transmission connecting device.
In addition, a rotating shaft of the photoreceptor driving motor 29 extends through a hole formed in the plate 27, and securingly mounts a photoreceptor motor gear 34A to an end thereof. Furthermore, a photoreceptor driving gear 36A is rotatably mounted on the plate 27. Moreover, an intermediate gear 35B is rotatably mounted on the plate 27 to engage with the photoreceptor motor gear 34A and photoreceptor driving gear 36A. In addition, the photosensitive roller 7 that is an image carrying body is connected to the photoreceptor driving gear 36A through a transmission connecting device.
In this construction, in order to miniaturize the image forming apparatus, the developing roller 16 and the photosensitive roller 7 are reduced in outer diameter, and the developing roller driving gear 33 and photoreceptor driving gear 36A are correspondingly reduced.
With the arrangement of the prior art described above, however, there is a limitation in arranging the two driving trains in the same plane with compactness, so that the developing roller driving train and photosensitive roller driving train are driven separately by the expensive motors 28 and 29 for the reason that it is not possible to arrange a shaft, which bears thereon a gear, on a rotating gear.
In this regard, due to the necessity for mounting the two motors in non-interfering positions, it is not possible to arrange the respective driving trains in positions which permit driving forces to be transmitted shortest distances from the motor shafts to the respective rollers 7 and 16, so that the driving forces must be transmitted from the motor shafts to the respective rollers via a roundabout way with the use of the intermediate gears 35A and 35B which are likely to cause uneven rotation.
In this manner, with the prior image forming apparatus, it is necessary to provide the motor 28 for driving the developing roller 16 and the motor 29 for driving the photosensitive roller 7, respectively, which leads to increased cost.
In addition, since the two motors 28 and 29 must be mounted in positions, where interfering therebetween is avoided, to drive the developing roller 16 and photosensitive roller 7, respectively, it is necessary to use the intermediate gears 35A and 35B that are likely to generate uneven rotation and are inferior in space efficiency.